1. Technical Field
This invention relates generally to capacitive sensors, and more particularly to a capacitive force sensor employing a capacitor pair disposed on a substrate to determine an amount of contact force.
2. Background Art
Portable electronic devices, including mobile telephones and personal digital assistants (PDAs), are becoming more sophisticated as the technology associated with these devices has continued to advance. For example, until recently, the user interface for a mobile telephone included twelve to fifteen mechanical buttons. Today, however, many portable devices include full QWERTY keypads. Further, some portable electronic devices employ touch sensitive screens that have no mechanical keys at all. Rather than pressing a mechanical button, the user simply touches a plastic or glass surface. Touch sensing circuits such as capacitive touch sensors then determine the location of the user's finger or stylus. A processor then correlates the position of the finger or stylus with a virtual keypad to determine keystrokes. Such capacitive sensors are described, for example, in commonly assigned, copending U.S. patent application Ser. No. 11/679,228, filed Feb. 27, 2007, entitled “Adaptable User Interface and Mechanism for a Portable Electronic Device,” which is incorporated herein by reference.
While capacitive touch sensors are useful for determining the geometric location of objects like fingers and styluses, they do not provide an indication of the contact force being applied. In many devices, force sensitive resistors or switches must be used to detect contact force. However, when the user interface comprises only a fixed piece of glass or plastic, force sensitive resistors exhibit some less desirable properties. For example, force sensitive resistors have a generally non-linear response that is dependent upon material and manufacturing tolerances. Additionally, force sensitive resistors are costly, thereby increasing the cost of the overall electronic device.
There is thus a need for an improved, lower cost touch sensitive interface that is capable of detecting and determining a contact force.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.